Hypomagnesemia is commonly seen but frequently overlooked in critically ill patients in intensive care unit (ICU). However, the strength and consistency of the effect of hypomagnesemia on outcomes in critically ill patients remain controversial. In this report, we performed a systematic review and meta-analysis to evaluate the association of serum magnesium level with prognosis of critically ill patients upon admission to the ICU. A comprehensive search for clinical trials was performed, and 10 studies comprising 1,122 cases and 630 controls were finally selected for analysis. The patients with hypomagnesemia had higher mortality rate (risk ratio [RR] 1.76; 95% confidence interval [CI] 1.54–2.00; P <0.00001), more frequently had sepsis (RR 2.04; 95% CI 1.21–3.42; P = 0.0007) and more frequent need for ventilatory support (RR 1.36; 95% CI 1.21 to 1.53; P <0.00001). Length of ICU stay was also higher in the hypomagnesemia group (RR 1.85; 95% CI 0.43– 3.26; P = 0.01). Collectively, our data indicated that hypomagnesemia appears associated with greater risk of mortality, sepsis, mechanical ventilation, and the length of ICU stay in patients admitted to ICU. The role of magnesium therapy for improving outcomes in critically ill patients is needed to further study.